Power systems typically include multiple, distinct frequency control levels. Primary frequency control is typically implemented locally at each generator and works autonomously, usually within one to three seconds after a disturbance. Droop controllers, which adjust speed or output of the generator, are one example of primary frequency control. Secondary frequency control includes Automatic Generation Control (AGC), in which a central controller adjusts the active power output of multiple generators in an area to restore the frequency and power interchanges with other control areas to their target values. Secondary frequency control typically occurs within four seconds to five minutes after a disturbance. Tertiary frequency control response typically takes minutes to hours and is used to address non-urgent events and longer-lasting effects or for other reasons.
The integration of renewable generation and distributed energy resources (DERs) into the electric system continues at a fast pace and is posed to be a permanent trend. These resources are typically coupled to the grid using power electronics rather than rotating generators. The reality of more inverter-coupled generation, in addition to related trends, is resulting in decreased system inertia and posing challenges toward stabilizing grid voltage and frequency.